This invention relates generally to very fast setting cementitious compositions that can be used for a variety of applications in which rapid hardening and attainment of early strength is desirable. In particular, the invention relates to those cementitious compositions that can be used to make boards with excellent moisture durability for use in wet and dry locations in buildings. Precast concrete products such as cement boards are made under conditions which provide a rapid setting of the cementitious mixture so that the boards can be handled soon after the cementitious mixture is poured into a stationary or moving form or over a continuously moving belt. Ideally, this will be occur as soon as 10 minutes, but more practically, setting of the cement mixture may be achieved up to about 20 minutes after being mixed with a suitable amount of water.
In U.S. Pat. No. 4,488,909, Galer et al discuss cementitious compositions capable of such rapid setting. Their compositions permit high speed production of carbon dioxide resistant cement boards by forming essentially all of the potential ettringite within about 20 minutes after the composition is mixed with water. The essential components of their cementitious composition are portland cement, high alumina cement, calcium sulfate and lime. Pozzolans such as fly ash, montmorillonite clay, diatomaceous earth and pumicite may be added up to about 25%. The cement composition includes about 14 to 21 wt % high alumina cement, which in combination with the other components makes possible the early formation of ettringite and other calcium aluminate hydrates, which are responsible for the early setting of the cementitious mixture.
In general, Galer's rapid setting formulation suffers from several major limitations. These limitations, as highlighted below, are even more of a concern for the production of cementitious products such as cement boards:                The final setting times of the cementitious mixtures are typically greater than 9 minutes. The final setting time is defined further in the examples below, but more generally, the cementitious mixtures have set to the extent that the cement-based products made thereof can be handled and stacked, although chemical reactions may continue for extended periods.        The amount of high alumina cement in the reactive powder blend is very high. Typically, the high alumina cement is greater than 14 wt % of the reactive powder blend.        The amount of pozzolanic materials is limited to 25 wt % of the reactive powder blend.        Lime is required as an additional ingredient to obtain rapid set. Presence of excess lime in cement boards is detrimental to their long-term durability performance. Cement boards often are reinforced with polymer coated glass fiber mesh that degrades, losing strength and ductility in a high alkaline environment. Presence of excess lime increases the alkalinity of the cementitious matrix and thereby negatively impacts the long-term durability performance of the polymer coated glass fiber mesh and the resulting cement boards. In addition, presence of excess lime also makes the concrete susceptible to sulfate attack thus influencing its durability.        
Ettringite is a compound of calcium aluminum sulfate compound having the formula:Ca6Al2(SO4)3.32 H2Oor alternatively:3 CaO.Al2O33 CaSO4.32 H2O
Ettringite forms as long needle-like crystals and provides rapid early strength to cement boards, so that they can be handled soon after being poured into a mold or over a continuous casting and forming belt. In the Galer et al compositions, alumina required for the formation of ettringite is supplied by high alumina cement (HAC) that typically contains 36-42 wt % Al2O3. Sulfate ions necessary for the formation of ettringite are provided by the gypsum present in Portland cement plus added gypsum, which is normally soluble in water. The lime needed for the formation of ettringite is provided by the portland cement and the added lime. The HAC is less soluble and is typically present in excess of that needed for ettringite formation. Thus, the gypsum and added lime are substantially consumed in the formation of ettringite. The availability and consumption of the HAC is usually increased by using a finely ground material. Since an excess of HAC is present in the cement board, it would be desirable to reduce its concentration as HAC is one of the more expensive components in the mixture.
High alumina cement has an advantage over portland cement since HAC develops its maximum strength much earlier. However, it does not retain that strength over time as secondary reactions occur. HAC does not set very rapidly by itself, but its contribution to the formation of ettringite and other calcium aluminate hydrates in the presence of other reactive powder blend components makes HAC valuable in production of cement boards. However, the amount of HAC used in the cement boards should be limited to what is necessary for forming ettringite in quantities sufficient to allow handling of the cement boards. Since HAC is more expensive than portland cement, the cost of making the cement boards can be reduced substantially if HAC is limited to smaller amounts, or more preferably even eliminated.
In the Galer et al composition, about 72 to 80 wt % is portland cement, about 14 to 21 wt % is HAC, about 3.5 to 10 wt % is calcium sulfate, and about 0.4 to 0.7 wt % is hydrated lime. Other materials, such as pozzolanic materials, e.g. fly ash, superplastizers, and other cement additives may be included.
Alkanolamines have been proposed for reducing the set time of cements, for example see U.S. Pat. Nos. 2,437,842; 3,553,077; 4,257,814 and 4,741,782. The effect of triethanolamine on the hydration of cement has been reported in technical papers, including Cement and Concrete Research, vol. 6, pp. 623-632, 1976, Pergamon Press, Inc. Such published information was more generally concerned with hydration of cement, rather than the preparation of cement boards and other precast concrete products, such as those of Galer et al., in which very rapid setting is needed. In their invention, Galer et al., provided the aluminates using high alumina cement (HAS) and sulfate ions using gypsum needed to form ettringite and achieve rapid setting of their cementitious mixture.
To achieve rapid set and final setting times of the order described in the Galer et al compositions, the current state-of-the-art inherently assumes that calcium aluminate cements such as HAC are required as a necessary and fundamental component in cementitious mixtures exhibiting rapid setting characteristics.
Thus, it was an objective of the present inventors to develop a cementitious composition that is capable of developing an extremely rapid set while simultaneously satisfying the following conditions:                The reactive powder blend of the cementitious composition should contain low concentrations of high alumina cement. Preferably, high alumina cement should be completely eliminated as a component of the reactive powder blend. Reducing alumina cement content helps to lower the cost of the product since high alumina cement is the most expensive component of a cementitious composition.        The reactive powder blend of the cementitious composition should contain very high concentrations of mineral additives, such as pozzolanic materials, up to 55 wt % of the reactive powder blend. Increasing the content of mineral additives, e.g. fly ash, would help to substantially lower the cost of the product. Moreover, use of pozzolanic materials in the composition would also help to enhance the long-term durability of the product as a consequence of the pozzolanic reactions.        The reactive powder blend of the cementitious composition should be free of externally added lime. Reduced lime content would help to lower the alkalinity of the cementitious matrix and thereby increase the long-term durability of the product.        The final setting time as measured according to the Gilmore needle (i.e., the time after which cement boards can be handled) of the cementitious composition preferably should be less than 60 minutes, more preferably less than 20 minutes and most preferably less than 10 minutes. A shorter setting time would help to increase the production output and lower the product manufacturing cost.        